This feature was available only for EC2 resources.
https://support.n2ws.com/portal/kb/articles/cpm-supports-custom-encryption-keys-for-dr
To enable this option for RDS and Aurora, please upgrade to
latest 2.X.X version
Reference defect number – N2WS-3093, fixed in v2.3.0e
Link to Release Notes and the latest patch – https://support.n2ws.com/portal/kb/articles/release-notes-for-the-latest-v2-3-x-cpm-release